Recently published experiments suggest that in the B\lymphocyte, membrane-bound immunoglobulin (mIg) is associated with other plasma membrane proteins as well as with the cytoskeleton. The objective of this work is to elucidate the molecular nature of the mIg interactions and to determine how they are influenced when mIg is either cross-linked or capped as a result of anti-Ig reagents binding to B-cell surfaces. Recently, we have found that in the chicken at least some cytoskeletally-associated Ig is phosphorylated. Under non-reducing conditions, the phorphorylated Ig is a dimer of one light chain and one heavy chain, with only the heavy chain phosphorylated. While cross linking mIg with anti-Ig reagents will link additional mIg to the cytoskeleton, newly linked Ig does not become phosphorylated. However, cross linking mIg will trigger the phosphorylation of other proteins. In particular, proteins with molecular weights of 130 and 38 kilodaltons appear to show enhanced tryosine phosphorylation under these conditions. Both of these proteins bind to actin and so may be associated with the cytoskeleton. Their phosphorylation may represent a significant step in B-cell activation. In our system, even mIg that does not co-isolate with the cytoskeleton may be bound to actin via disulphide linkages, appearing on the surface as H2L2(actin)2 complexes with an Ig heavy chain of 80 kilodaltons. However, we also find evidence for mIg present on the surface in larger disulphide-linked structures with Ig heavy chains, all with a molecular weight of 77 kilodaltons. The 77 kilodalton mIg heavy chain may be the more significant with respect to antigen binding because of an apparent high avidity. At this point, we do not know the relationship of this form of mIg with the cytoskeleton. (I)